Not Applicable
Not Applicable
1. Field of the Invention
This invention relates generally to a venturi flowmeter, and more particularly to a venturi flowmeter for use in an exhaust sampling apparatus.
2. Description of Related Art
Under present day federal regulations, the exhaust emissions from motor vehicles must not exceed specified values of certain constituent contaminants, as set forth in the Code of Federal Regulations. See Title 40 of the Code of Federal Regulations, Parts 81-99, Subparts A, B, D, E, F, G, K and N. See also Kaufman U.S. Pat. No. 3,699,814. The presence of such standards has made it imperative that the exhaust emissions from vehicle engines be tested and analyzed to determine the relative amount of certain constituents therein. Much effort has gone into the development of equipment for use in this field of exhaust gas sampling, and it is now known to deliver exhaust gases from an internal combustion engine at an accurately controlled flow rate through a test apparatus for purposes of determining and analyzing the relative amounts of constituents therein. The general scheme of such testing is to add dilution air to the exhaust gases. The total volume of the mixture of exhaust and dilution air must be measured. A continuously proportional sample of volume must be collected and is stored for subsequent analysis of constituents such as hydrocarbons, carbon monoxide, and NO.sub.x. Mass emissions are determined from the sample concentrations and total flow over the test period.
One such system for analyzing samples from exhaust gases is set forth in U.S. Pat. No. is 3,699,814 to Kaufman entitled, xe2x80x9cGas Sampler,xe2x80x9d issued Oct. 24, 1972. The Kaufaman patent, the disclosure of which is incorporated herein by reference, taught a much improved gaseous exhaust emission sampler which replaced the constant displacement pump of prior systems with a critical flow venturi and centrifugal blower for metering the diluted exhaust emissions at a constant volume flow.
Another system utilizes a pair of critical flow venturis for proportional sampling. An example of such a system is set forth in U.S. Pat. No. 3,817,100. In another such system, a downstream pump produces a sufficient vacuum on the bulkstream critical flow venturi exit so that the bulkstream mixture is flowing at sonic velocity, a condition which limits the bulkstream mixture to a constant mass flow rate at a given set of upstream temperature and pressure conditions measured at the bulkstream critical flow venturi inlet. A sample is extracted from the dilute bulkstream flow through another critical flow venturi in close proximity to the bulkstream critical flow venturi so that the venturis are operating under the same inlet pressure and temperature conditions. This sample critical flow venturi operates in connection with a downstream pump in the sampling line to create sonic flow, and thereby a constant mass flow rate at the measured upstream temperature and pressure conditions. Thus, the sample critical flow venturi extracts a sample for analysis at a flow rate proportional to the bulkstream flow rate.
Although the flow rate controlled by a critical flow venturi will vary a small amount due to changes in the venturi inlet temperature and pressure, if the sample venturi and bulkstream venturi are operating at critical flow rate conditions at identical inlet pressure and temperature conditions, the sample flow rate is extracted in proportion to the bulkstream flow rate.
The state of the art in this field is represented by Lewis et al., U.S. Pat. Nos. 5,184,501 and 6,122,980, and Lewis, U.S. Pat. Nos. 4,586,367, 4,660,408, and 4,823,591 (xe2x80x9cthe Lewis referencesxe2x80x9d). The Lewis references teach various embodiments of an exhaust sampling apparatus that use venturi flowmeters to measure the flow of an exhaust gas through the apparatus. However, the venturi flowmeters disclosed in the Lewis references do not measure reverse flow rates, so are therefore never positioned adjacent to the exhaust source, as in the present invention.
The prior art, including the Lewis references, also teaches the calculation of an average pressure; and the square root of the average pressure is then used to calculate the average flow rate. As described herein, this methodology is acceptable using prior art systems; however, this methodology introduces unacceptable errors when the venturi metering device is positioned adjacent to the exhaust source.
The prior art does teach a flowmeter that can measure reverse flow rates. Ortiz, U.S. Pat. No. 5,886,267, teaches a measuring system for measuring the flow of a liquid through a conduit such as an irrigation system. The system includes pressure transducers in an arrangement similar to the present invention. However, the Ortiz device is designed for use in a different field, and therefore does not teach many of the structures of the present invention. Ortiz does not teach the inclusion of temperature sensors for calculating the mass flow rate of an exhaust gas. Ortiz also does not teach the use of heaters for preventing the flow of liquids through the flowmeter.
The prior art teaches various venturi metering devices for measuring the flow rate of a gas through the device. However, the prior art does not teach a venturi metering device adapted for measuring the flow rate of a pulsing exhaust gas immediately adjacent an exhaust source. The present invention fulfills these needs and provides further related advantages as described in the following summary.
The present invention teaches certain benefits in construction and use which give rise to the objectives described below.
The present invention provides an apparatus for sampling the emission content of an exhaust gas from an exhaust source. The apparatus includes an exhaust inlet adapted for connecting the exhaust source with a venturi metering device for measuring the flow rate of the exhaust gas. The venturi metering device includes a housing having an entrance section, a throat section, and an exit section, the throat section having a smaller diameter than the entrance and exit sections. The housing includes a pressure sensing means for sensing the pressure in the throat section and the entrance section, and for producing an electrical output that characterizes the pressure that the pressure sensing means senses, the electrical output enabling a computer of the venturi metering device to accurately measure the pulsating flow rate of the exhaust gas in both directions through the housing.
A primary objective of the present invention is to provide an apparatus for sampling the emission content of an exhaust gas from an exhaust source, the apparatus having advantages not taught by the prior art.
Another objective is to provide a venturi metering device that can take rapid, accurate readings of the flow rate of an exhaust gas without introducing errors caused by the pulsation of the exhaust gas often found adjacent the exhaust source.
A further objective is to provide a more accurate method of calculating the average flow rate of the exhaust gas.
Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.